arXiv subjects

Troels Arnfred Bojesen

Publications and source records attributed to Troels Arnfred Bojesen.

10 recordsLinked to original sources

arXiv2023

Inductive Bias for Emergent Communication in a Continuous Setting

We study emergent communication in a multi-agent reinforcement learning setting, where the agents solve cooperative tasks and have access to a communication channel. The communication channel may consist of either discrete symbols or continuous variables. We introduce an inductive bias to aid with the emergence of good communication protocols for continuous messages, and we look at the effect this type of inductive bias has for continuous and discrete messages in itself or when used in combination with reinforcement learning. We demonstrate that this type of inductive bias has a beneficial effect on the communication protocols learnt in two toy environments, Negotiation and Sequence Guess.

cs.LG↗

arXiv2018

Policy Guided Monte Carlo: Reinforcement Learning Markov Chain Dynamics

We introduce \textit{Policy Guided Monte Carlo} (PGMC), a computational framework using reinforcement learning to improve Markov chain Monte Carlo (MCMC) sampling. The methodology is generally applicable, unbiased and opens up a new path to automated discovery of efficient MCMC samplers. After developing a general theory, we demonstrate some of PGMC's prospects on an Ising model on the kagome lattice, including when the model is in its computationally challenging kagome spin ice regime. Here, we show that PGMC is able to automatically machine learn efficient MCMC updates without a priori knowledge of the physics at hand.

physics.comp-ph↗

arXiv2017

Quantum spin ice under a [111] magnetic field: from pyrochlore to kagomé

Quantum spin ice, modeled for magnetic rare-earth pyrochlores, has attracted great interest for hosting a U(1) quantum spin liquid, which involves spin-ice monopoles as gapped deconfined spinons, as well as gapless excitations analogous to photons. However, the global phase diagram under a [111] magnetic field remains open. Here we uncover by means of unbiased quantum Monte-Carlo simulations that a supersolid of monopoles, showing both a superfluidity and a partial ionization, intervenes the kagomé spin ice and a fully ionized monopole insulator, in contrast to classical spin ice where a direct discontinuous phase transition takes place. We also show that on cooling, kagomé spin ice evolves towards a valence bond solid similar to what appears in the associated kagomé lattice model [S. V. Isakov et al., Phys. Rev. Lett. 97, 147202 (2006)]. Possible relevance to experiments is discussed.

cond-mat.str-el↗

arXiv2016

Topological semimetals protected by off-centered symmetries in nonsymmorphic crystals

Topological semimetals have energy bands near the Fermi energy sticking together at isolated points/lines/planes in the momentum space, which are often accompanied by stable surface states and intriguing bulk topological responses. Although it has been known that certain crystalline symmetries play an important role in protecting band degeneracy, a general recipe for stabilizing the degeneracy, especially in the presence of spin-orbit coupling, is still lacking. Here we show that a class of novel topological semimetals with point/line nodes can emerge in the presence of an off-centered rotation/mirror symmetry whose symmetry line/plane is displaced from the center of other symmorphic symmetries in nonsymmorphic crystals. Due to the partial translation perpendicular to the rotation axis/mirror plane, an off-centered rotation/mirror symmetry always forces two energy bands to stick together and form a doublet pair in the relevant invariant line/plane in momentum space. Such a doublet pair provides a basic building block for emerging topological semimetals with point/line nodes in systems with strong spin-orbit coupling.

cond-mat.mtrl-sci↗

arXiv2016

Lattices of double-quanta vortices and chirality inversion in $p_x+ip_y$ superconductors

We investigate the magnetization processes of a standard Ginzburg-Landau model for chiral p-wave superconducting states in an applied magnetic field. We find that the phase diagram is dominated by triangular lattices of doubly quantized vortices. Only in close vicinity to the upper critical field, the lattice starts to dissociate into a structure of single-quanta vortices. The degeneracy between states with opposite chirality is broken in a nonzero field. If the magnetization starts with an energetically unfavorable chirality, the process of chirality-inversion induced by the external magnetic field results in the formation of a sequence of metastable states with characteristic magnetic signatures that can be probed by standard experimental techniques.

cond-mat.supr-con↗

arXiv2015

Fluctuation effects in phase-frustrated multiband superconductors

We compare the phase-diagrams of an effective theory of a three-dimensional multi-band superconductor obtained within standard and cluster mean-field theories, and in large-scale Monte Carlo simulations. In three dimensions, mean field theory fails in locating correctly the positions of the phase transitions, as well as the character of the transitions between the different states. A cluster mean-field calculations taking into account order-parameter fluctuations in a local environment improves the results considerably for the case of extreme type-II superconductors where gauge-field fluctuations are negligible. The large fluctuations in the multi-component superconducting order parameter originate with strong frustration due to interband Josephson-couplings. A novel chiral metallic phase found in previous works using large scale Monte-Carlo computations, is not obtained either within the single-site mean-field theory or the improved cluster mean-field theory of order parameter fluctuations. In three-dimensional superconductors, this unusual metallic phase originates with gauge-field fluctuations.

cond-mat.supr-con↗

arXiv2014

Phase transitions and anomalous normal state in superconductors with broken time reversal symmetry

Using Monte Carlo simulations, we explore the phase diagram and the phase transitions in $\groupUZ$ $n$-band superconductors with spontaneously broken time-reversal symmetry (also termed $s+is$ superconductors), focusing on the three-band case. In the limit of infinite penetration length, the system under consideration can, for a certain parameter regime, have a single first order phase transition from a $\groupUZ$ broken state to a normal state due to a nontrivial interplay between $\groupU$ vortices and $\groupZ$ domain walls. This regime may also apply to multicomponent superfluids. For other parameters, when the free energy of the domain walls is low, the system undergoes a restoration of broken $\groupZ$ time reversal symmetry at temperatures lower than the temperature of the superconducting phase transition.{We show that inclusion of fluctuations can strongly suppress the temperature of the $\groupZ$-transition when frustration is weak. The main result of our paper is that} for relatively short magnetic field penetration lengths, the system has a superconducting phase transition at a temperature lower than the temperature of the restoration of the broken $\groupZ$ symmetry. Thus, there appears a new phase which is $\groupU$-symmetric, but breaks $\groupZ$ time reversal symmetry, an anomalous dissipative (metallic) state.

cond-mat.supr-con↗

arXiv2014

Josephson-frustrated superconductors in a magnetic field

We study the effect of an externally imposed rotation or magnetic field on frustrated multiband superconductors/superfluids. The frustration originates with multiple superconducting bands crossing the Fermi surface in conjunction with interband Josephson-couplings with a positive sign. These couplings tend to frustrate the phases of the various components of the superconducting order parameter. This in turn leads to an effective description in terms of a $\groupUZ$-symmetric system, where essentially only the $\groupU$-sector couples to the gauge-field representing the rotation or magnetic field. By imposing a large enough net vorticity on the system at low temperatures, one may therefore reveal a resistive vortex liquid state which will feature an unusual additional phase transition in the $\groupZ$-sector. At low enough vorticity there is a corresponding vortex-lattice phase featuring a $\groupZ$ phase transition. We argue that this Ising transition phase should be readily observable in experiments.

cond-mat.supr-con↗

arXiv2013

Berry phases, current lattices, and suppression of phase transitions in a lattice gauge theory of quantum antiferromagnets

We consider a lattice model of two complex scalar matter fields $z_{a}, a=1,2$ under a CP1 constraint $\abs{z_1}^2+\abs{z_2}^2=1$, minimally coupled to a compact gauge field, with an additional Berry phase term. This model has been the point of origin for a large body of works addressing novel paradigms for quantum criticality, in particular spin-quark (spinon) deconfinement in S=1/2 quantum antiferromagnets. We map the model exactly to a link-current model, which permits the use of classical worm algorithms to study the model in large-scale Monte Carlo simulations on lattices of size L^3, up to L=360. We show that the addition of a Berry phase term to the lattice $\CP$-model suppresses the phase transition in the $\groupO{3}$ universality class of the $\CP$-model. The link-current formulation of the model is useful in identifying the mechanism by which the phase transition is suppressed.

cond-mat.stat-mech↗

arXiv2013

Multihistogram Reweighting for Nonequilibrium Markov Processes Using Sequential Importance Sampling Methods

We present a multihistogram reweighting technique for nonequilibrium Markov Chains with discrete energies. The method generalizes the single histogram method of Yin et al. [Phys. Rev. E72, 036122 (2005)], making it possible to calculate the time evolution of observables at a posteriori chosen couplings based on a set of simulations performed at other couplings. In the same way as multihistogram reweighting in an equilibrium setting improves the practical reweighting range as well as use of available data compared to single histogram reweighting, the method generalizes the multihistogram advantages to nonequilibrium simulations. We demonstrate the procedure for the Ising model with Metropolis dynamics, but stress that the method is generally applicable to a range of models and Monte Carlo update schemes.

cond-mat.stat-mech↗